1. Field of the Invention
This invention relates generally to the field of engine retarders and more particularly to engine retarders wherein the exhaust valves of the engine are opened near the top dead center on the compression stroke of the engine so that the energy absorbed by the engine during the compression stroke is not returned to the engine during the expansion stroke. Such an engine retarder is known as a compression release engine retarder. The present invention relates specifically to a variable timing mechanism for an engine retarder of the above type.
2. Prior Art
For many years it has been recognized that the ordinary wheel braking mechanisms, commonly of the disc or drum type fitted to commercial vehicles, while capable of absorbing a large amount of energy during a short period, are incapable of absorbing the somewhat lesser amounts of energy required during an extended period of time, for example, during descent of a long but gradual decline. In such circumstances, the friction material used in the brake mechanism will become overheated (causing "brake fading") and may be destroyed while the metal parts may warp or buckle. In general, the problem has been resolved either by using a lower gear ratio so that the engine can function more effectively as a brake due to its internal friction or by employing some form of an auxiliary braking system. A number of such auxiliary braking systems, generally known as engine retarders, have been developed by the art, including hydrokinetic retarders, exhaust brakes, electric brakes, and gas compression release retarders. In each of these systems, a portion of the kinetic energy of the vehicle is transformed into heat as a result of gas compression, fluid friction, or electrical resistance and, thereafter, dissipated to the atmosphere directly or through the vehicle exhaust or cooling system. The common characteristic of such auxiliary braking systems is the ability to absorb and dissipate a certain amount of power continuously or at least for an indefinite but relatively long period of time.
The hydrokinetic and electric retarders are generally quite heavy and bulky since they require turbine or dynamo mechanisms and thus may be undesirable from the viewpoint of initial cost as well as operating cost. The exhaust brake, while generally simple and compact, necessarily increases the exhaust manifold pressure and may occasion "floating" of the exhaust valves of the engine, a generally undesirable condition.
It has long been recognized that in the ordinary operation of an internal combustion engine employing the Otto or Diesel cycle, for example, a considerable amount of work is done during the compression stroke upon the air or air/fuel mixture introduced into the cylinder. During the expansion or power stroke of the engine the work of compression is recovered so that, neglecting friction losses, the net work due to compression and expansion is zero and the net power output is that resulting from the combustion of the air/fuel mixture. When the throttle is closed or the fuel supply interrupted, the engine will, of course, function as a brake to the extent of the friction inherent in the engine mechanism.
Many attempts have been made to increase the braking power of an engine by converting the engine into an air compressor and dumping the compressed air through the exhaust system. A simple and practical method of accomplishing this end is disclosed in Cummins U.S. Pat. No. 3,220,392. In that patent an auxiliary exhaust valve actuating means synchronized with the engine crankshaft is provided which opens the exhaust valve near the end of the compression stroke, without interfering with the normal actuating cam means for the exhaust valve, together with appropriate control means for the auxiliary exhaust valve actuator. While the engine retarding means set forth in detail in the Cummins U.S. Pat. No. 3,220,392 is capable of producing a retarding power approaching the driving power of the engine under normal operating conditions, experience with this mechanism has revealed that the retarding power may be affected significantly by the timing of the opening of the engine exhaust valve.
If the exhaust valve is opened too late a significant portion of the retarding power may be lost due to the expansion of the compressed air during the initial part of the expansion stroke. On the other hand, if the exhaust valve is opened too early, there may be insufficient compression during the compression stroke which, similarly, will reduce the amount of retarding power that can be developed.
The timing of the exhaust valve opening is affected to a significant degree by the temperature conditions in the engine which vary as a result of changes in operating conditions. It will be appreciated, for example, that the length of the engine exhaust valve stem will increase with increases in temperature, thereby reducing clearance or "lash" in the exhaust valve actuating mechanism, i.e., the exhaust valve train. While it is known to provide adjustable elements in the valve actuating mechanism by means of which the clearance may be set (see, for example, U.S. Pat. No. 3,220,392, FIG. 2, element 301), the clearance as determined by the rocker arm adjusting screw (or equivalent element) must be at least large enough when the engine is cold so that some clearance will remain when the engine is hot. If there is inadequate clearance when the engine is hot, the exhaust valve may be held in a partially open condition. In this circumstance, the operations of the engine may be affected adversely and the exhaust valves are apt to be burned. To avoid such effects, it is common to provide a clearance on the order of 0.018 inch in the exhaust valve actuating mechanism.
In Custer U.S. Pat. No. 4,398,510 a timing advance mechanism is disclosed which automatically changes the valve train lash from the engine operating mode value, i.e., 0.018 inch cold adjustment, to a lesser or negative amount when the engine is in the retarding mode. The hydro-mechanical mechanism of U.S. Pat. No. 4,398,510 is incorporated into the slave piston adjusting screw and comprises an hydraulic piston which automatically extends a predetermined distance from the adjusting screw body whenever the engine is placed in the retarding mode and high pressure is generated in the retarder hydraulic system. The mechanism of U.S. Pat. No. 4,398,510 is capable of modifying the exhaust train cold clearance by any particular predetermined amount and this increases the retarding horsepower developed by the engine, the increase being greater at higher engine speeds.
Since the development of the mechanism of U.S. Pat. No. 4,398,510, truck operators have sought to decrease the level of pollutants emitted by the internal combustion engine and to increase the fuel economy of the engine by de-tuning the engine and lowering the engine speed. Although these engine operating conditions are effective for their intended purposes, they reduce the operating effectiveness of the compression release engine retarder. As a result, a need is presented for an engine retarder with improved retarding performance.